1. Field of the Invention
The present invention relates to a power supply system, an electric vehicle with the power supply system mounted thereon, and a method of regulating an amount of fuel supply. More particularly the present invention pertains to a technique of regulating an amount of fuel supply to fuel cells included in a power supply system with the fuel cells and a storage battery.
2. Description of the Prior Art
A proposed power supply system includes fuel cells and storage batteries as power sources, wherein the fuel cells charge the storage batteries and the storage batteries charged to a sufficient level supply electric power to a loading (for example, JAPANESE PATENT LAYING-OPEN GAZETTE No. 6-124720). This power supply system includes a plurality of storage batteries, one of which is connected to the loading while the other is connected to the fuel cells. The storage battery having a less remaining charge is charged by the fuel cells, while the other storage battery supplies electric power to the loading. This structure ensures the sufficient charge state of the storage battery connected to the loading, thereby stably supplying electric power to the loading, such as a driving motor of the electric vehicle.
Since the proposed power supply system has a plurality of storage batteries, however, a relatively large space is required for installation of the power supply system. Especially when the power supply system is mounted on a vehicle as a power source of a motor for driving the vehicle, the relatively bulky power supply system in the restricted space of the vehicle undesirably reduces the degree of freedom in design of the vehicle. Only one storage battery is connected to the loading at one time. Each of the storage batteries is thus required to have a marginal capacity, in order to ensure sufficient outputs in case of an abrupt increase in loading, for example, at the time of starting the vehicle or going up a slope. The increase in capacity of the storage battery, however, results in increasing the weight. In some cases, it is accordingly impossible to mount a plurality of storage batteries having a sufficient capacity on the vehicle.
An improved structure has been proposed to reduce the size of the conventional power supply system that has a plurality of storage batteries and enables the storage batteries to be switched and alternately supply electric power to the loading. This improved structure has fuel cells and a storage battery, which are connected in parallel and enables at least either one of the fuel cells and the storage battery to supply electric power to the loading. In case that the loading is smaller than a predetermined level and the fuel cells have a marginal output, the fuel cells can charge the storage battery while driving the loading. In case that the loading is greater than a predetermined level, on the other hand, both the fuel cells and the storage battery work to drive the loading. This structure decreases the required capacity of the storage battery, thereby reducing the size of the power supply system.
In the power supply system of this structure that enables reduction in size, however, the charging condition of the storage battery may cause shortage of gases supplied to the fuel cells, which lead to various troubles. In case that the storage battery is in the insufficient charge state, the fuel cells are expected to charge the storage battery while driving the loading. When the sufficient amounts of gases corresponding to the output from the fuel cells to the storage battery are not fed to the fuel cells, some problems arise in the process of power generation in the fuel cells.
In case that the amounts of gases supplied to the fuel cells are insufficient for the required output of the fuel cells, a voltage drop, for example, may occur in the fuel cells to damage their function as the power source. In case that the amounts of gases supplied to the fuel cells are insufficient for the amount of the connected loading, this causes a voltage drop as well as uneven power generation in the respective unit cells constituting the stack of fuel cells, which results in an abnormal phenomenon, such as a change of poles in part of the unit cells. The change of poles is a phenomenon that reverses the anode and cathode in the cell reactions. Such an abnormal phenomenon makes the voltage unstable and causes energy that has not been converted to electrical energy to be released as thermal energy and partially increase the temperature of the fuel cells. The partial temperature increase damages the elements of the fuel cells and shortens the life of the fuel cells.
Possible shortage of gases in the fuel cells due to the insufficient charge state of the storage battery is a significant problem especially at the time of starting the power supply system. It is assumed that the power supply system stops operation while the storage battery has an insufficient remaining charge. At the time of a next start of the power supply system, the fuel cells are required to start a supply of electric power to the storage battery simultaneously with a start of warm-up. Supply of only the required amounts of gases for the warm-up operation causes shortage of gases in the fuel cells.